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Homework Help: Total Elastic Potential Energy 2

  1. Jan 27, 2005 #1
    I need to know how to go about finding the speed.

    A block B of mass 5 kg is fastened to one end of each of two springs. The other ends of the springs are attached to fixed points A and C, 4 metres apart on a smooth horizontal surface, as shown in the diagram.

    Spring AB has natural length 2 metres and modulus of elasticity 30 N, while BC has natural length 1 metre and modulus 40 N (you may assume that the springs meet at the centre of B).

    If the block is moved 0.5 metres towards C from its equilibrium position and then released, determine its speed as it passes through its equilibrium position.

    this is what i have tried

    Energy in string when moved 0.5m = total mech energy before the 0.5 movement
    E= 30*(0.5)^2/(2*2) = 1.575 J

    KE= 1/2*5*v^2 = 5/2*v^2 J and
    E in string 30*(3/8)^2/2*2 = 7.93 J

    7.93 + 5/2v^2 = 1.875
    this leads me to taking square root of a -ve number

    could someone help me!

    thanks in advance
     
  2. jcsd
  3. Jan 28, 2005 #2
    I'm going to assume this is a follow-up to the question I did https://www.physicsforums.com/showthread.php?t=61316".

    The springs already had an extension at the equilibrium position, so when you move it box towards C, the extension will change.
    The extension in AB (x) is now: 0.5 + 8/11 = 27/22
    The extension in BC (y) is now: 3/11 - 0.5 = -5/22 (i.e. compression)

    Calculate the total mechanical energy at that point (which is only EPE). This should equal the total mechanical energy at the equilibrium position (which is KE & EPE).

    It's important that you calculate the total energy. You shouldn't get the square root of a negative number. :smile:
     
    Last edited by a moderator: Apr 21, 2017
  4. Jan 28, 2005 #3
    cheers i can dig that
     
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